Color cathode ray picture tubes, including those used in home entertainment television receivers and those used in medium-resolution and high-resolution tubes intended for CRT monitors, have a front assembly which includes a panel. The panel includes a grille formed on its interior face, a shadow mask, and means for supporting the shadow mask.
The grille is formed through a process which deposits a black matrix on the interior face of the panel. The process involves the application of polyvinyl alcohol (PVA)/sensitizer, a light-sensitive material, to the panel. Once the PVA has been applied, the panel is exposed to ultra-violet light which is passed through holes in a shadow mask. Once exposed to the ultra-violet light, the panel is rinsed in water and the PVA is washed away from areas that were not exposed to the ultra-violet light. Thereafter, a graphite solution is applied to the panel and dried to become graphite material, followed by hydrogen peroxide which attacks the PVA. The loosened graphite material is rinsed off, thereby leaving the black matrix pattern which will separate the phosphors which are applied to the panel in a later screening process.
Since the grille (i.e., black matrix) is formed by light passing through the shadow mask, it follows that the production of a nondefective grille is directly dependent on a nondefective mask. Although each shadow mask is thoroughly cleaned before the exposure step of the grille forming process, there is no way to assure that the mask holes will not become plugged in transit from the cleaning step to the exposure step. Since the mask contains approximately 800,000 holes, they can become plugged with an object as minute as a dust particle.
In effect, if any of the mask holes become plugged, the ultra-violet light is not allowed to pass therethrough and the PVA which has been applied to the panel therefore will not be exposed. The ultimate result is a defective grille including an excess amount of graphite material in an area designated for phosphor adhesion. In the industry, defective high-resolution grilles account for approximately 1 to 40% of the total volume of grilles produced.
Once a defective grille has been identified, it is customary in the industry to either wash out the panel so that it can be re-grilled, or to attempt to remove the excess graphite material from the grille.
The presently exercised means by which the excess graphite material is removed from the grille comprises the use of a microscope through which a worker views the defect, and the use of a needle or the like with which the worker can scrape or scratch away the excess material.
Clearly, there is a need in the industry for an efficient, cost-saving apparatus which will cure such defects on grilles of color cathode ray tubes.